Solid-state imaging device, manufacturing method of solid-state imaging device and electronic apparatus

ABSTRACT

A solid-state imaging device, includes: plural unit pixels including a photoelectric conversion portion converting incident light into an electrical signal, and a waveguide having a quadratic curve surface at an inner surface and introducing the incident light to the photoelectric conversion portion.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional of U.S. patent application Ser.No. 12/660,290, filed Feb. 24, 2010, which claims priority from JapanesePatent Application No. JP 2009-050132 filed in the Japanese PatentOffice on Mar. 4, 2009, all of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid-state imaging device, amanufacturing method of the solid-state imaging device and an electronicapparatus, and particularly relates to a solid-state imaging devicehaving a waveguide structure, a manufacturing method of the waveguidestructure and an electronic apparatus having the solid-state imagingdevice.

2. Description of the Related Art

A solid-state imaging device, for example, a CMOS-type solid-stateimaging device (hereinafter, referred to as “CMOS image sensor”) has afeature in which peripheral circuits such as a DSP (Digital SignalProcessor) can be mounted on the same chip (substrate) by utilizing aCMOS process. When peripheral circuits are mounted on the same circuit,there is a case where a multilevel interconnection structure such asfour-layered structure concerning wiring is used for reducing the scaleof peripheral circuits.

However, since the distance between a substrate surface (siliconinterface) to a micro lens (on-chip lens) is increased in the case wherethe multilevel interconnection structure is used, light condensingefficiency when light which is incident through the micro lens iscondensed (introduced) on a light receiving surface of a light receivingportion (photoelectric conversion portion) is reduced. When the lightcondensing efficiency is reduced, pixel sensitivity is reduced.

Accordingly, a so-called waveguide structure is known, in which awaveguide is provided at the center of the pixel, thereby confininglight within the waveguide to reduce the light amount loss in an opticalpath leading to the light receiving portion from the micro lens in orderto increase the light condensing efficiency and to improve pixelsensitivity.

In related art, in order to introduce light to the light receivingportion more efficiently, a waveguide structure having a forward-taperedshape portion in which the size of a planer shape seen from a lightincident direction is decreased gradually from a surface of the lightincident side to the light receiving portion side is proposed (forexample, refer to JP-A-2004-221532 (Patent Document 1). Moreover, astructure in which an aperture at an upper portion of the tapered shapeportion (entrance side) of a tapered shape portion is largely opened tothereby increase the incident light amount as compared with thewaveguide structure described in Patent Document 1 is proposed (forexample, refer to JP-A-2008-103757 (Patent Document 2)).

SUMMARY OF THE INVENTION

In cases of the waveguide structures described in Patent Documents 1, 2,light can be condensed on the light receiving surface of the lightreceiving portion efficiently by the action of the tapered shapeportion, however, light obtained when light perpendicular to the lightreceiving surface (light incident in parallel to the central axis of thewaveguide) is reflected on the tapered surface of the waveguide is notconsidered. That is, it is difficult to condense light reflected in thewaveguide on the light receiving surface of the light receiving portionin the structure.

In view of the above, it is desirable to provide a solid-state imagingdevice which can efficiently condense light including light incident inparallel to the central axis of the waveguide on the light receivingportion (photoelectric conversion portion), a manufacturing method ofthe waveguide structure and an electronic apparatus having thesolid-state imaging device.

According to an embodiment of the invention, in a solid-state imagingdevice having plural unit pixels including a photoelectric conversionportion converting incident light into an electrical signal and awaveguide introducing the incident light to the photoelectric conversionportion, an inner surface of the waveguide is a quadratic curve surface.

Since the inner surface of the waveguide is the quadratic curve surfacein the solid-state imaging device having the above structure,particularly when light incident in parallel to the central axis of thewaveguide (parallel light) is reflected on the quadratic curve surface,the light is efficiently condensed to the photoelectric conversionportion due to characteristics included in the quadratic curve surface,as a result, light condensing efficiency is increased.

According to embodiments of the invention, particularly, light incidentin parallel to the central axis of the waveguide can be efficientlycondensed to the photoelectric conversion portion and light condensingefficiency can be increased, therefore, the sensitivity can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system configuration view showing an outline of aconfiguration of a CMOS image sensor to which the invention is applied;

FIG. 2 is a circuit diagram showing an example of a circuitconfiguration of a unit pixel;

FIG. 3 is a cross-sectional view showing a cross-sectional structure ofa pixel having the waveguide structure according to a first embodimentof the invention;

FIG. 4 is a view showing a state in which light entering in parallel toa symmetry axis “0” of a parabola is condensed to a focal point F1 ofthe parabola;

FIG. 5 shows process drawings showing an example of a manufacturingmethod of the waveguide structure according to the first embodiment;

FIG. 6 is a cross-sectional view showing a cross-sectional structure ofa pixel including a waveguide structure according to a second embodimentof the invention;

FIG. 7 is a view showing a state in which light passing through a focalpoint F1 is condensed to a focal point F2 in an ellipse;

FIG. 8 is a cross-sectional view showing a case in which the waveguidestructure according to the second embodiment is applied to the CMOSimage sensor using a pupil correction technique;

FIG. 9 shows process drawings showing an example of a manufacturingmethod of the waveguide structure according to the second embodiment;and

FIG. 10 is a block diagram showing a configuration example of an imagingapparatus which is an example of an electronic apparatus according to anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, best modes for carrying out the invention (hereinafter,written as “embodiments”) will be explained in detail with reference tothe drawings. The explanation will be made in the following order.

1. Solid-state imaging device to which the invention is applied (anexample of a CMOS image sensor)

2. First Embodiment (an example in which a waveguide inner surface is aparabolic surface)

3. Second Embodiment (an example in which a waveguide inner surface isan elliptical surface)

4. Modification example

5. Application example (imaging apparatus)

1. Solid-State Imaging Device to which the Invention is Applied

[System configuration]

FIG. 1 is a system configuration view showing an outline of aconfiguration of a solid-state imaging device, for example, a CMOS imagesensor which is a type of an X-Y address type solid-state imaging deviceto which the invention is applied. Here, the CMOS image sensor is animage sensor formed by applying or partially using a CMOS process.

As shown in FIG. 1, a CMOS image sensor 10 according to the applicationexample includes a pixel array unit 11 formed on a semiconductorsubstrate (chip) 18 and peripheral circuit units integrated on the samesemiconductor substrate 18 as the pixel array unit 11. As the peripheralcircuits, for example, a vertical drive unit 12, a column processingunit 13, a horizontal drive unit 14 and a system control unit 15 areprovided.

In the pixel array unit 11, not-shown unit pixels (hereinafter, they maybe written as merely “pixels”) including photoelectric conversion units(for example, photodiodes) which photoelectrically convert incidentvisible light into the charge amount corresponding to the light amountare two-dimensionally arranged in a matrix state. Each pixel is providedwith a lens for condensing incident light, namely, a so-called microlens, a color filter in the case of color display and the like thoughnot shown. A specific configuration of the unit pixel will be describedlater.

In the pixel array unit 11, pixel drive lines 16 are formed along theright and left direction in the drawing (pixel arrangement direction ofpixel rows/horizontal direction) with respect to respective rows in thepixel arrangement of the matrix state, and vertical signal lines 17 areformed along the up and down direction in the drawing (pixel arrangementdirection of pixel columns/vertical direction) with respect torespective columns. In FIG. 1, only one pixel drive line 16 is shown,however, it is not limited to one line. An end of the pixel drive line16 is connected to an output end corresponding to each row of thevertical drive unit 12.

The vertical drive unit 12 includes a shift register, an address decoderand the like. The vertical drive unit 12 includes a read scanning systemand a sweep scanning system though specific configurations thereof arenot shown here. The read scanning system sequentially performs selectivescanning of unit pixels from which signals are read row by row.

On the other hand, the sweep scanning system performs sweep scanningwhich sweeps (resets) unnecessary charges from the photoelectricconversion elements of unit pixels in a reading row to which readingscanning is performed by the read scanning system preceding the readscanning by a period of time corresponding to shutter speed. A so-calledelectronic shutter operation is performed by the sweeping (resetting) ofunnecessary charges by the sweep scanning system. The electronic shutteroperation means operations of sweeping photoelectric charges of thephotoelectric conversion elements and newly starting exposure (startingaccumulation of photoelectric charges).

A signal read by the reading operation by the read scanning systemcorresponds to the light amount which has been incident after theimmediately preceding reading operation or the electronic shutteroperation. A period from the read timing by the immediately precedingreading operation or the sweep timing by the immediately precedingelectronic shutter operation until the read timing by the readingoperation at this time will be an accumulation time (exposure time) ofphotoelectric charges in the unit pixel.

Signals outputted from respective unit pixels of the pixel rowsselectively scanned by the vertical drive unit 12 are supplied to thecolumn processing unit 13 through respective vertical signal lines 17.The column processing unit 13 performs predetermined signal processingto analog pixel signals outputted from respective pixels 20 in theselected rows by each pixel column in the pixel array unit 11.

As the signal processing in the column processing unit 13, for example,a CDS (Correlated Double Sampling) processing is cited. The CDSprocessing is processing in which reset levels and signal levelsoutputted from respective pixels of the selected row and taking thedifference of these levels to thereby obtain pixel signals of one rowand remove fixed pattern noise of pixels. An AD conversion functionwhich digitalizes analog pixel signals can be also included in thecolumn processing unit 13.

The horizontal drive unit 14 includes a shift register, an addressdecoder and the like, sequentially performing selective scanning circuitportions corresponding to pixel columns of the column processing unit13. According to the selective scanning by the horizontal drive unit 14,pixel signals processed in the column processing unit 13 by each pixelcolumn are sequentially outputted.

The system control unit 15 receives clocks given from the outside of thesemiconductor substrate 18, data instructing operation modes and thelike, and further outputs data such as internal information of the CMOSimage sensor 10. The system control unit 15 further includes a timinggenerator which generates various timing signals, performing drivecontrol of the vertical drive unit 12, the column processing unit 13 andthe horizontal drive unit 14 and the like based on various timingsignals generated by the timing generator.

[Circuit Configuration of the Unit Pixel]

FIG. 2 is a circuit diagram showing an example of the unit pixel 20. Asshown in FIG. 2, the unit pixel 20 according to the circuit exampleincludes four transistors, for example, a transfer transistor 22, areset transistor 23, an amplification transistor 24 and a selectiontransistor 25, in addition to a photodiode 21 which is an example of thephotoelectric conversion unit.

In this case, as four transistors 22 to 25, for example, an N-channelMOS transistor is used. However, the combination of conductive types inthe transfer transistor 22, the reset transistor 23, the amplificationtransistor 24 and the selection transistors 25 which are cited here isjust an example, and it is not limited to the combination.

As the pixel drive line 16, for example, three drive wires of a transferline 161, a reset line 162 and a selection line 163 are provided withrespect to each pixel in the same pixel row in common in the unit pixel20. Respective ends of the transfer line 161, the reset line 162 and theselection line 163 are connected to output ends corresponding to eachpixel row of the vertical drive unit 12 by each pixel row.

The photodiode 21 is connected to a negative-side power supply (forexample, a ground) at an anode electrode, photoelectrically convertsreceived light into photoelectric charges (photoelectrons, in this case)of the charge amount corresponding to the light amount and accumulatesthe photoelectric charges. A cathode electrode of the photodiode iselectrically connected to a gate electrode of the amplificationtransistor 24 through the transfer transistor 22. A node 26 which iselectrically connected to the gate electrode of the amplificationtransistor 24 is called a FD (floating diffusion) portion.

The transfer transistor 22 is connected between the cathode electrodeand the FD portion 26 of the photodiode 21. To a gate electrode of thetransfer transistor 22, a transfer pulse φTRF in which the high level(for example, Vdd level) is active (hereinafter, written as “Highactive”) is given through the transfer line 161. According to this, thetransfer transistor 22 becomes ON state and transfers photoelectriccharges which have been photoelectrically converted by the photodiode 21to the FD portion 26.

The reset transistor 23 is connected to a pixel power supply Vdd at adrain electrode and connected to the FD portion 26 at a sourceelectrode, respectively. To a gate electrode of the reset transistor 23,a high-active reset pulse φRST is given through a reset line 162.According to this, the reset transistor 23 becomes ON state and resetsthe FD portion 26 by sweeping charges in the FD portion 26 to the pixelpower supply Vdd before signal charges are transferred from thephotodiode 21 to the FD portion 26.

The amplification transistor 24 is connected to the FD portion 26 at agate electrode and connected to the pixel power supply Vdd at a drainelectrode, respectively. The amplification transistor 24 outputs apotential of the FD portion 26 after being reset by the reset transistor23 as a reset signal (reset level) Vreset. The amplification transistor24 further outputs the potential of the FD portion after the transfer ofsignal charges by the transfer transistor 22 is outputted as a lightaccumulation signal (signal level) Vsig.

The selection transistor 25 is connected to a source electrode of theamplification transistor 24 at a drain electrode and connected to thevertical signal line 17 at a source electrode. To a gate electrode ofthe selection transistor 25, a high-active selection pulse φSEL is giventhrough a selection line 163. According to this, the selectiontransistor 25 becomes ON state and relays signals outputted from theamplification transistor 24 to the vertical signal line 17 with the unitpixel 20 as a selection state.

The selection transistor 25 can apply a circuit configuration of beingconnected between the pixel power supply Vdd and the drain of theamplification transistor 24.

The unit pixel 20 is not limited to the pixel configuration includingfour transistors in the above configuration. For example, a pixelconfiguration including three transistors in which the amplificationtransistor 24 doubles as the selection transistor 25 can be applied andany pixel circuit configuration can be used.

The CMOS image sensor 10 according to the application example of theinvention explained as the above includes a waveguide structure in whicha waveguide is provided at the center of the pixel in order to introducelight incident on each pixel 20 efficiently into the photodiode 21(namely, to increase light condensing efficiency).

According to an embodiment of the invention, the CMOS image sensor 10having the waveguide structure can further improve the light condensingefficiency at the time of introducing light incident through the microlens to a light receiving surface of the photodiode 21. A specificembodiment of the waveguide structure for improving the light condensingefficiency will be explained as follows.

2. First Embodiment

[Pixel Structure]

FIG. 3 is a cross-sectional view showing a cross-sectional structure ofa pixel having the waveguide structure according to a first embodimentof the invention.

In FIG. 3, a photoelectric conversion portion (light receiving portion)32 which photoelectrically converts incident light is formed on asurface layer portion of a semiconductor substrate corresponding to thesemiconductor substrate 18 of FIG. 1, for example, a silicon substrate31. The photoelectric conversion portion 32 is formed as a diode(corresponding to the photodiode 21 in FIG. 2) including, for example, aP-type diffusion layer and an N-type diffusion layer on the substratesurface side of the P-type diffusion layer. The photoelectric conversionportion 32 may have a structure in which the surface thereof is furthercovered by a hole accumulation layer including the P-type diffusionlayer.

A gate electrode 33 of the pixel transistor such as the transfertransistor 22 is formed through a base insulating film (not shown) andan interlayer insulating film 34 is deposited on the silicon substrate31. On the surface side of the interlayer insulating film 34, wirings 35are formed by burying a conductive material in groove patterns. Thedeposition of the interlayer insulating film 34 and the formation ofwiring 35 are repeated, then, the interlayer insulating film 34 isfinally formed to thereby form a multilevel interconnection layer 36.

In the multilevel interconnection layer 36, the wirings 35 at respectivelayers are electrically connected through a contact portion 37appropriately. At a portion above the photoelectric conversion portion32 in the multilevel interconnection layer 36, a hole for the waveguide38A is formed with a parabolic surface (paraboloid). A lighttransmissive buried layer (transparent film) 39 is buried inside thehole 38A to form a waveguide 40A. Here, the waveguide 40A is formed bythe transmissive buried layer 39 being buried in the hole 38A formedwith the barabolic surface, therefore, an inner surface of the waveguide40A becomes the parabolic surface.

On the multilevel interconnection layer 36 including the waveguide 40A,a color filter 43 of a given color 43 is formed through a passivationfilm 41 and the planarization film 42. Further on the color filter 43, alens, specifically, a micro lens 44 called an on-chip lens is provided.The above components constitute the unit pixel 20 having the waveguide40A which introduces light incident through the micro lens 44 to thelight receiving surface of the photoelectric conversion portion 32.

In the pixel including the waveguide 40A having the above configuration,the parabolic surface forming the inner surface of the waveguide 40A isa kind of a quadratic curve surface. It is known that light (parallellight) entering in parallel to a symmetry axis “O” of a parabola whichis a cross section of the parabolic surface is condensed to a focalpoint F1 of the parabola as shown in FIG. 4. A feature of the embodimentis a point in which the shape of the parabola is used in the innersurface of the waveguide 40A by utilizing the principle of the parabola.

The waveguide 40A is preferably provided so that the central axis of thewaveguide 40A corresponding to the symmetry axis “O” of the parabolacorresponds to one light ray passing through the center of the microlens 44, namely, a main light ray P. The lens has a surface which isrotational symmetry around one axis, and the rotational symmetry axiswill be an optical axis. A point where the rotational symmetry axis anda lens sphere cross each other will be the center of the micro lens 44.

The waveguide 40A according to the embodiment is formed so that thefocal point F1 in the parabola is positioned on, for example, the lightreceiving surface of the photoelectric conversion portion 32 (interfaceof the photodiode 21). The position of the focal point F1 in theparabola is not limited to the light receiving surface of thephotoelectric conversion portion 32.

As described above, the inner surface of the waveguide 40A is made to bethe parabolic surface, thereby condensing light on the light receivingsurface of the photoelectric conversion portion 32, which has not beencondensed on the light receiving surface of the photoelectric conversionportion 32 by the micro lens 44, particularly, light (parallel light)incident in parallel to the central axis (optical axis) P of thewaveguide 40A. Therefore, the light condensing efficiency can beimproved as compared with the structure in related art which has atapered shape at the inner surface, in which light reflected in thewaveguide could not be condensed on the light receiving surface of thephotoelectric conversion portion 32.

When the distance between pixels is reduced especially along withminiaturization of the pixel size in recent years, in the waveguidestructure in related art which has the waveguide structure of thetapered shape at the inner surface, light transmitted through thewaveguide is incident on the photoelectric conversion portion of anadjacent pixel and photoelectrically converted in the adjacent pixels,which will be part of the reason for color mixture. On the other hand,in the waveguide structure according to the embodiment, light incidenton the waveguide 40A can be efficiently condensed on the light receivingsurface of the photoelectric conversion portion 32, therefore, colormixture caused by light transmitted through the waveguide 40A can beprevented.

The above structure can further condense light parallel to the opticalaxis P on the light receiving surface of the photoelectric conversionportion 32, therefore, the structure can be applied to a pixel structurenot having the micro lens 44. Moreover, since the inner surface of thewaveguide 40A is the parabolic surface, the size of the planer shapeseen from the light incident direction is larger on the side of themicro lens 44 than on the side of the photoelectric conversion portion32. Therefore, the structure has an advantage in which light transmittedthrough the micro lens 44 can be taken more into the waveguide 40A ascompared with the waveguide structure having, for example, a cylindricalshape.

The embodiment has the waveguide structure in which the focal point F1in the parabola is positioned on the light receiving surface of thephotoelectric conversion portion 32, however, the position of the focalpoint F1 in the optical axis direction can be optionally set accordingto design. There is no problem on the photoelectric conversion if theposition of the focal point F1 of the parabola is below the lightreceiving surface (interface) of the photoelectric conversion portion32, therefore, it is possible to take more margin concerning theposition of the focal point F1 as well as to condense light other thanthe parallel light on the light receiving surface of the photoelectricconversion portion 32.

Furthermore, the position of the focal point F1 can be changed accordingto colors, namely, red light, green light and blue light. According tothis, light condensing upon considering more efficient photoelectricconversion according to colors can be realized. Specifically, a regionin which more efficient photoelectric conversion can be performed(photoelectric conversion region) in the photoelectric conversionportion 32 differs according to wavelengths of light. Concerning thephotoelectric conversion region in the photoelectric conversion portion32, the region of the blue light is the shallowest and the region of thered light is the deepest.

Therefore, the position (depth) of the focal point F1 from the lightreceiving surface (interface) of the photoelectric conversion portion 32is set so as to be deeper in the order of blue light, green light andred light. According to this, the light condensing upon considering moreefficient photoelectric conversion can be realized according torespective colors of red light, green light and blue light. Thewaveguide structure in related art including the waveguide structurehaving the tapered shape at the inner surface is made only byconsidering the condensing of incident light on the light receivingsurface, not having the structure suitable to respective colors in viewof the photoelectric conversion efficiency.

The size of a beam spot condensed to the focal point F1 can beoptionally set by changing, for example, curvature of the parabolicsurface. Even when the size of the beam spot is changed, the lightamount is not changed. Therefore, the beam spot can be reliably appliedto the light receiving surface by reducing the size of the beam spot,for example, even when the light receiving surface of the photoelectricconversion portion 32 is reduced along with the miniaturization of thepixel size in recent years.

The size of the beam spot can be changed according to respective colorsof red light, green light and blue light. The size of the beam sport ischanged according to respective colors, thereby aiming at thephotoelectric conversion region optimum for each color when applying thebeam spot even if the photoelectric conversion region in thephotoelectric conversion portion 32 differs according to respectivecolors as described above. As a result, more efficient photoelectricconversion according to respective colors of red light, green light andblue light can be realized. Moreover, since the beam spot can bereduced, the beam spot can be applied to the photoelectric conversionregion while avoiding obstacles.

[Manufacturing Method]

Subsequently, an example of processes in a manufacturing methodconcerning pixels having the waveguide structure according to the abovefirst embodiment will be explained with reference to process drawings inFIG. 5. In order to make understanding easier, the same signs are givenin FIG. 5 to the same components as components of FIG. 3.

Here, processes of the waveguide structure according to the firstembodiment, namely, processes of forming the waveguide 40A in theinterlayer insulating film 34 of FIG. 3 will be explained. Knownprocesses can be used for processes before and after the processes offorming the waveguide structure.

Processes in which the aspect ratio differs in the front half and thelatter half are used in the course of processes of forming the hole 38Afor the waveguide. Furthermore, etching is performed repeatedly whilechanging power of an etching gas, thereby forming the inner surface ofthe convex hole 38A as described below.

Specifically, as tangents of the parabola are straight lines, etching isperformed by using a photoresist 51A having a hole size corresponding toa bottom surface of the hole for waveguide 38A (Process 1). Next,etching is performed by using a photoresist 51B having a hole sizelarger than the hole size of the photoresist 51A (Process 2).Subsequently, etching is performed by using a photoresist 51C having ahole size further larger than the hole size of the photoresist 51B(Process 3).

As explained above, processes of performing etching by graduallyincreasing the hole size (in stages or continuously) while allowing thephotoresist 51 to move back will be repeatedly executed the number oftimes enough to form the parabolic surface in plural stages. Accordingto this, the hole for waveguide 38A having the parabolic surface at theinner surface is eventually formed (Process N). In the etchingprocesses, etching proceeds not only in the transversal direction butalso in the depth direction, therefore, the etching surface can bedeformed into an arbitrary shape by increasing the number of times ofetching and changing power of the etching gas.

3. Second Embodiment

[Pixel Structure]

FIG. 6 is a cross-sectional view showing a cross-sectional structure ofa pixel including a waveguide structure according to a second embodimentof the invention, and the same signs are given in the drawing to thesame components of FIG. 3.

In FIG. 6, a photoelectric conversion portion 32 which photoelectricallyconverts incident light is formed on a surface layer portion of asilicon substrate 31. The photoelectric conversion portion 32 is formedas a diode (corresponding to the photodiode 21 in FIG. 2) including, forexample, a P-type diffusion layer and an N-type diffusion layer on thesubstrate surface side of the P-type diffusion layer. The photoelectricconversion portion 32 may have a structure in which the surface thereofis further covered by a hole accumulation layer including the P-typediffusion layer.

A gate electrode 33 of the pixel transistor such as the transfertransistor 22 is formed through a base insulating film (not shown) andan interlayer insulating film 34 is deposited on the silicon substrate31. On the surface side of the interlayer insulating film 34, wirings 35are formed by burying a conductive material in groove patterns. Thedeposition of the interlayer insulating film 34 and the formation ofwiring 35 into groove patterns are repeated, then, the interlayerinsulating film 34 is finally formed to thereby form a multilevelinterconnection layer 36.

In the multilevel interconnection layer 36, the wirings 35 at respectivelayers are electrically connected through a contact portion 37appropriately. At a portion above the photoelectric conversion portion32 in the multilevel interconnection layer 36, a hole for the waveguide38B is formed with an elliptical surface. A light transmissive buriedlayer (transparent film) 39 is buried inside the hole 38B to form awaveguide 40B. Here, the waveguide 40B is formed by the transmissiveburied layer 39 being buried in the hole 38B formed with the ellipticalsurface, therefore, an inner surface of the waveguide 40B becomes theelliptical surface.

On the multilevel interconnection layer 36 including the waveguide 40B,a color filter of a given color 43 is formed through a passivation film41 and the planarization film 42. Further on the color filter 43, amicro lens 44 is provided. The above components constitute the unitpixel 20 having the waveguide 40B which introduces light incidentthrough the micro lens 44 to the light receiving surface of thephotoelectric conversion portion 32.

In the pixel including the waveguide 40B having the above configuration,the elliptical surface forming the inner surface of the waveguide 40B isa kind of a quadratic curve surface. It is known that light transmittedthrough a focal point F1 is condensed to a focal point F2 in an ellipsewhich is a cross section of the elliptical surface as shown in FIG. 7. Afeature of the embodiment is a point in which the shape of the ellipseis used in the inner surface of the waveguide 40B by utilizing theprinciple of the ellipse.

The waveguide 40B is preferably provided so that the focal point F1positioned on the central axis of the waveguide 40B corresponding to thesymmetry axis “O” of the ellipse is positioned at on a main light ray Ppassing through the center of the micro lens 44. The main light ray Pindicates one light ray passing through the center of the micro lens 44,namely, a point where the rotational symmetry axis of the micro lens 44and a lens sphere cross each other as described above. The micro lens 44condenses incident light to the focal point F1 (focal point F1 of theellipse) of the waveguide 40B.

There is no problem on the photoelectric conversion whether the focalpoint F2 (focal point F2 of the ellipse) of the waveguide 40B ispositioned on the light receiving surface (interface) of thephotoelectric conversion portion 32 or whether it is positioned belowthe light receiving surface of the photoelectric conversion portion 32in the same manner as the first embodiment. Moreover, the position ofthe focal point F2 can be changed according to colors, namely, redlight, green light and blue light, thereby realizing light condensingupon considering more efficient photoelectric conversion according tocolors.

The size of the beam spot condensed to the focal point F2 can be changedaccording to colors in the same manner as the first embodiment, therebyrealizing more efficient photoelectric conversion according torespective colors in the photoelectric conversion portion 32.

As described above, the inner surface of the waveguide 40B is made to bethe elliptical surface, thereby condensing light condensed by the microlens 44 and passes through the focal point F1 on the light receivingsurface of the photoelectric conversion portion 32 or on the focal pointF2 positioned below the light receiving surface. Therefore, the lightcondensing efficiency can be improved as compared with the waveguidestructure in related art including the waveguide structure having thetapered shape at the inner surface.

[Pupil Correction]

The waveguide 40B according to the embodiment is particularly suitablefor being applied to a CMOS image sensor using a known technique calleda pupil correction which is generally used to a thin-type camera lens anexit pupil distance of which is short for the purpose of alleviating thedecrease in the peripheral light amount.

The pupil correction is a technique as described below. First, in thecentral portion of the pixel array unit 11 to be an imaging surface, thecenter of the micro lens 44 taking light incident on the pixel 20individually is allowed to be aligned with the center of an aperture ofthe photoelectric conversion portion 32 (namely, a region barycenter ofthe photoelectric conversion portion 32). On the other hand, in theperipheral portion of the pixel array unit 11, the central position ofthe micro lens 44 is shifted (an offset is provided) with respect to thecenter of the aperture of the photoelectric conversion portion 32 so asto correspond to the direction of the main optical ray toward theoutside. Here, the main light ray indicates one light ray passingthrough the center of the micro lens 44 as described above.

In short, the technique of providing an offset between the center of themicro lens 44 and the aperture center of the photoelectric conversionportion 32 in the peripheral portion of the pixel array unit 11 iscalled the pupil correction. Using the technique of the pupil correctionallows the camera lens to be thinner and prevents vignetting of light atthe aperture periphery of the photoelectric conversion portion 32 evenif incident light to the micro lens 44 enters the imaging surface atvarious angles. According to this, light condensing efficiency ofrespective pixels 20 can be approximately constant all over the pixelarray unit 11 (entire imaging surface), therefore, comprehensiveperformance improvement in sensitivity can be realized.

In the CMOS image sensor using the technique of the pupil correction,the offset is generated between the center of the micro lens 44 and theaperture center of the photoelectric conversion portion 32 at theperipheral portion of the pixel array unit 11 as described above. Then,in the waveguide structure in related art including the waveguidestructure having the tapered shape at the inner surface, it is highlylikely that light reflected in the waveguide deviates from the lightreceiving surface of the photoelectric conversion portion 32 to a largedegree, which leads to the reduction of sensitivity and shading.

On the other hand, in the CMOS image sensor using the technique of thepupil correction, the light condensing efficiency can be increased byusing the waveguide structure according to the embodiment, namely, thewaveguide structure in which the inner surface of the waveguide 40B isan elliptical surface as the waveguide structure of pixels at theperipheral portion of the pixel array unit 11. Specifically, lightincident on pixels obliquely is condensed to the focal point F1 by themicro lens 44 as shown in FIG. 8, thereby efficiently condensing lightpassing through the focal point F1 to the photoelectric conversionportion 32. Therefore, the sensitivity can be improved as compared withthe waveguide structure in related art and shading can be prevented.

[Manufacturing Method]

Subsequently, an example of processes in a manufacturing methodconcerning pixels having the waveguide structure according to the abovesecond embodiment will be explained with reference to process drawingsof FIG. 9. In order to make understanding easier, the same signs aregiven in FIG. 9 to the same components as components of FIG. 6.

Here, processes of the waveguide structure according to the secondembodiment, namely, processes of forming the waveguide 40B in theinterlayer insulating film 34 of FIG. 6 will be explained. Knownprocesses can be used for processes before and after the processes offorming the waveguide structure.

As processes of forming the hole for the waveguide 38B, the sameprocesses as processes of forming the hole for the waveguide 38A in thecase of the above first embodiment are basically used in the presentexample. Specifically, etching is performed by using a photoresist 51Ahaving a hole size corresponding to a bottom surface of the hole forwaveguide 38B (Process 1). Next, etching is performed by using aphotoresist 51B having a hole size larger than the hole size of thephotoresist 51A (Process 2).

Subsequently, etching is performed by using a photoresist 51C having ahole size further larger than the hole size of the photoresist 51B(Process 3). Accordingly, etching is performed by gradually increasingthe hole size (in stages or continuously) while allowing the photoresist51 to move back. These processes are repeatedly executed the number oftimes enough to form the half of the elliptical surface in pluralstages, thereby forming a semi-elliptical surface 38B-1 at the lowerside of the hole for the waveguide 38B having the elliptical innersurface (Process N).

Next, a semi-elliptical surface 38B-2 at the upper side is formed by thesame processes (Process 1 to Process N). In the etching processes,etching proceeds not only in the transversal direction but also in thedepth direction, therefore, the etching surface can be deformed into anarbitrary shape by increasing the number of times of etching andchanging power of the etching gas.

Then, a substrate in which the upper-side semi-elliptical surface 38B-2is formed is bonded in an upside-down state to a substrate in which thelower-side semi-elliptical surface 38B-1 is formed, thereby eventuallyforming the hole for the waveguide 38B having the elliptical surface atthe inner surface (Process N+1). As a technique of bonding twosubstrates, a known technique used in a back-illuminated image sensorcan be used. The bonding of the two substrates can be performed byusing, for example, a SOI (Silicon On Insulator) substrate.

4. Modification Example

In the above respective embodiments, the cases of the waveguidestructures in which the holes for the waveguide 38A, 38B are formed inthe interlayer insulating film 34 and the light transmissive buriedlayer 39 is buried into these holes 38A, 38B have been explained asexamples, however, the present invention is not limited to theapplication to the waveguide structure. For example, it is possible toapply the technique of making the inner surfaces of the holes for thewaveguide 38A, 38B be the quadratic curve surface also with respect tothe waveguide structure in which a metal film is formed at the innersurfaces of the holes for the waveguide 38A, 38B and light is reflectedby the metal film.

Also in the above respective embodiments, the cases of application tothe CMOS image sensor have been explained as examples, however, it isnot limited to the application to the CMOS image sensor. That is, theinvention can be applied to all X-Y address type solid-state imagingdevices in which unit pixels detecting charges corresponding to thelight amount of visible light as the physical quantity and outputs themas electrical signals are arranged in a matrix state. The invention canbe further applied to a charge-transfer type solid-state imaging devicetypified by a CCD (Charge Coupled Device) image sensor, not limited tothe X-Y address type solid-state imaging device.

The solid-state imaging device may be formed as one chip, or may beformed as a module mode having an imaging function in which an imagingunit and a signal processing unit or an optical system are integrallypackaged.

5. Electronic Apparatus

The invention is not limited to the application to the solid-stateimaging device but can be applied to electronic apparatuses such as animaging apparatus. Here, electronic apparatuses indicate imagingapparatuses (camera systems) such as a digital still camera and a videocamera, mobile devices such as a cellular phone and a PDA (PersonalDigital Assistant) having an imaging function. The module mode to bemounted on the electronic apparatus, namely, a camera module is dealt asthe imaging apparatus.

[Imaging Apparatus]

FIG. 10 is a block diagram showing a configuration example of an imagingapparatus as an example of an electronic apparatus according to anembodiment of the invention. As shown in FIG. 10, an imaging apparatus100 according to the embodiment has an optical system including a lensgroup 101 and the like, an imaging device 102, a DSP circuit 103 whichis a camera signal processing unit, a frame memory 104, a display device105, a recording device 106, an operation system 107, a power supplysystem 108 and the like. The DSP circuit 103, the frame memory 104, thedisplay device 105, the recording device 106, the operation system 107and the power supply system 108 are mutually connected through a busline 109.

The lens group 101 takes incident light (image light) from a subject andbrings light into focus on an imaging surface of the imaging device 102.The imaging device 102 converts the light amount of incident lightfocused on the imaging surface by the lens group 101 into electricalsignals in respective pixels and outputs the signals as pixel signals.As the imaging device 102, the CMOS image sensor in which plural pixelshaving the waveguide structure according to the first and secondembodiments are arranged is used.

The display device 105 includes a panel-type display device such as aliquid crystal display device and an organic EL (electro luminescence)display device, displaying moving pictures or still pictures imaged bythe imaging device 102. The recording device 106 records moving picturesor still pictures imaged by the imaging device 102 in recording mediasuch as a video tape and a DVD (Digital Versatile Disk).

The operation system 107 issues operation commands of various functionsincluded in the imaging apparatus under the control by a user. The powersupply system 108 appropriately supplies various power supplies to beoperation power supplies for the DSP circuit 103, the frame memory 104,the display device 105, the recording device 106 and the operationsystem 107 to these supply targets.

The imaging device 100 is applied to a video camera or a digital stillcamera, and camera modules for mobile devices such as a cellular phone.In the imaging apparatus 100, the CMOS image sensor according to thefirst and second embodiments is used as the imaging device 102, therebyimproving light condensing efficiency of the CMOS image sensor andimproving the sensitivity, as a result, the imaging apparatus havingexcellent image quality can be provided. Since the miniatulization ofthe pixel size can be realized due to the improvement of thesensitivity, therefore, high-definition imaged pictures corresponding tothe increase of pixels can be provided.

The present application contains subject matter related to thatdisclosed in Japanese Priority Patent Application JP 2009-050132 filedin the Japan Patent Office on Mar. 4, 2009, the entire contents of whichis hereby incorporated by reference.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

What is claimed is:
 1. A solid-state imaging device, comprising: pluralunit pixels including a photoelectric conversion portion convertingincident light into an electrical signal, and a waveguide having aquadratic curve surface at an inner surface and introducing the incidentlight to the photoelectric conversion portion, wherein the quadraticcurve surface is an elliptical surface.
 2. The solid-state imagingdevice according to claim 1, wherein the unit pixel includes a microlens taking the incident light, and the micro lens condenses theincident light to one focal point of the elliptical surface.
 3. Thesolid-state imaging device according to claim 2, wherein the center ofthe micro lens is arranged with an offset with respect to the aperturecenter of the photoelectric conversion portion.
 4. The solid-stateimaging device according to claim 1, wherein the unit pixel includes acolor filter, and a position of the focal point of the parabolic surfaceor the other focal point of the elliptical surface in an optical axisdirection differs according to colors of the color filter.
 5. Thesolid-state imaging device according to claim 1, wherein the unit pixelincludes a color filter, and a spot size of light condensed to the focalpoint of the parabolic surface or the other focal point of theelliptical surface differs according to colors of the color filter.